The invention concerns a receiver for radio-frequency signals, in particular of the GPS type, having means for correcting the effects of multipath signals. The invention also concerns a method for activating or setting the receiver into operation.
The receiver for radio-frequency signals modulated by specific codes of transmitting sources includes receiving and shaping means. These means allow frequency conversion of the radio-frequency signals to provide intermediate signals.
The receiver also includes a correlation stage formed of several correlation channels which receive the intermediate signals. Each channel is provided with a correlator in which the intermediate signals are correlated. This correlation is achieved by means of at least one control loop of the correlator, when the channel is being used, with at least two replicas of the specific code of a visible transmitting source to be searched and tracked, which are in phase early and late. The correlator includes means for integrating the correlated signals to provide, at the end of each integration period, a first amplitude value of the auto-correlation function of the early signals and a second amplitude value of the auto-correlation function of the late signals. In a transmitting source tracking mode, the first and second amplitude values are kept substantially equal.
The receiver also includes microprocessor means connected to the correlation stage for processing the data extracted from the radio-frequency signals after correlation.
If said receiver is a GPS receiver, the data extracted from the radio-frequency signals is, in particular, the GPS message, the pseudo-ranges and the Doppler frequency, this data being used to calculate the position, velocity and time (hour).
The radio-frequency signal receiver of the present invention can of course also be used in a satellite navigation system of the GLONASS or GALILEO type. Likewise, the receiver could be used in a mobile telephone network, for example of the CDMA type (Code-division multiple access). In such case, the transmitting sources are no longer satellites but base cells of the telephone network, and the processed data concerns audible or legible messages, or navigation messages.
Currently, 24 satellites are placed in orbit at a distance close to 20,200 km above the surface of the Earth on 6 orbital planes each offset by 55° with respect to the equator. The time taken by a satellite to make a complete revolution in orbit before returning to the same point above the Earth is approximately 12 hours. The distribution of the satellites in orbit allows a terrestrial GPS receiver to receive GPS signals from at least four visible satellites to determine its position, velocity and the local time.
In civil applications, each of the satellites in orbit transmits radio-frequency signals formed of a carrier frequency L1 at 1.57542 GHz on which are modulated a pseudo-random PRN code at 1.023 MHz peculiar to each satellite and a GPS message at 50 Hz. The GPS message contains the ephemerides and almanac data of the transmitting satellite, which are useful in particular for calculating the X, Y, Z position, velocity and time related data.
The PRN code (pseudo random noise), in particular of the Gold code type, is different for each satellite. This Gold code is a digital signal formed of 1023 chips which are repeated every millisecond. This repetition period is also defined by the term “epoch” of the Gold code. It is to be noted that a chip takes the values 1 or 0 as for a bit. However, a chip (the term used in GPS technology) is to be differentiated from a bit which is used to define a unit of data.
The Gold codes defined for 32 satellite identification numbers have the characteristic of being orthogonal. By correlating them with each other the correlation result gives a value close to 0. This characteristic thus enables several radio frequency signals transmitted on a same frequency originating from several satellites simultaneously to be independently processed in several channels of the same GPS receiver.
Currently, in several daily activities, GPS receivers, which are portable or incorporated particularly in vehicles, are used to allow navigation data to be provided to users. This data facilitates, in particular, orientation, the search for a target and knowledge of bearings. Moreover, portable GPS receivers tend to be of smaller size so as to enable them also to be incorporated in objects, which can easily be transported by one person, such as in cellular telephones or in wristwatches. However, as they are powered by a battery or accumulator of small size, it is often necessary to minimise the energy consumed by the receivers.
It should be noted that a GPS receiver needs to pick up the radio-frequency signals transmitted by at least four visible satellites in order to determine in particular its position and time related data. The receiver can also pick up the ephemerides data and almanac data peculiar to each satellite by locking on individually to one of the visible satellites.
FIG. 1 shows schematically GPS receiver 1, provided with an antenna 2 for picking up radio-frequency signals. Said GPS receiver 1 has to receive signals SV1 to SV4 originating from at least four visible satellites S1 to S4 in order to be able to determine its position, velocity and time-related data. However, when said receiver 1 is used in locations surrounded by various obstacles, such as buildings B in towns, certain radio-frequency signals SV1′ and SV3′ picked up by receiver 1 are sometimes reflected across these obstacles B. These signals SV1′ and SV3′ reflected and combined with the direct signals SV1 and SV3 which originate from the same transmitting source can cause errors as to the data extracted from the set of signals picked up by the receiver. These errors have, in particular, repercussions on the calculation of the receiver's position.
Phase errors due to multipath signals can be greater than or equal to 150 ns for terrestrial navigation receivers, which corresponds to an error of 45 m on the calculated position. Generally, nominal errors are within a margin of 30 ns, which corresponds to an error of approximately 9 m on a calculated position. These errors are usually difficult to remove completely, even though this phenomenon of multipath signals is well known. Several embodiments have already been proposed to minimise the effect of such multipath signals.
One may cite, in particular, Patent Application No. WO 95/14937 of the Novatel company, which discloses a pseudo-random noise encoded radio-frequency signal receiver provided with means for compensating distortions due to multipath signals. In order to do this, the receiver includes several correlation channels, which are each intended to acquire a specific satellite at the same time. Auto-correlation means for each channel include several correlators, which each receive an internally generated pseudo-random code replica phase shifted with respect to another replica to be correlated with intermediate signals. An output signal power level estimator for each correlator of a channel is provided to eliminate the effects of the multipath signals. The phase shift between each replica is for example less than 0.2 chips which requires a high establishment frequency for each replica.
A major drawback of this receiver is that each channel is provided with a multitude of correlators for the acquisition and tracking phases of a specific visible satellite. Consequently, the very high number of elements needed to form the correlation stage of the channels leads to high energy consumption, which does not allow the receiver to be integrated in a portable object including a low capacity energy source.
In a same technical context, U.S. Pat. No. 5,966,403 of Trimble Navigation Limited discloses a spread spectrum radio-frequency receiver which also includes means for minimising the effects of multipath signals. This document proposes two alternative embodiments. In the first variant, a uniform or non-uniform signal weighting function is used for correlating the intermediate signals with early and late replicas. The microprocessor means receive several correlated and weighted signals, and close the carrier and code control loops. These microprocessor means have the task of estimating the signal distortion due to multipath signals and of minimising such distortion.
In a second variant, two correlation channels of the receiver are used in parallel for tracking the same satellite whose transmitted signals are capable of being diverted from their trajectory. The second channel is used to allow the microprocessor means to minimise distortion due to multipath signals. A phase delay is imposed for generating the early and late replicas of each channel in order for the microprocessor means to be able to evaluate the distortion due to multipath signals.